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Study Guide for Forces and Newton’s Laws of Motion
Forces:
 Know how gravity and friction affect the motion of objects.
o What affects gravity? ________________ and _____________________
o What things affect friction? Surface area, gravity pull, mass
 Understand how to calculate net force (sum of forces acting on an object) and how that
force affects the motion of objects
o Forces acting in same direction (working together) add them up, example: If
Bobby is pushing his bike up a hill with a force of 10N and another kid is pulling
Bobby’s bike up the hill at the same time with 15N of force the net force on the
bike is 25N (10 + 15=25)
o Forces acting in opposite directions (against each other) you subtract one from
the other. Example: Bobby is pushing his bike up the hill with 10N of force, but his
friend is pushing the bike down the hill at the same time with 15N of force, the
net force on the bike is 5N going down the hill. (15-10=5).
o IF NET FORCE = 0 - No change in motion!
 Example, you pull on a dog’s leash with 20N force toward your house, she
tugs back with 20N force away from your house… Net force =0 and you
don’t move
 Be able to distinguish between weight and mass. Which one changes because of
gravity?
o How do we calculate weight? Fg=mg Fg = m x (9.8)
 Unbalanced forces cause a change in motion in the direction the larger force is acting.
 Balanced or equal forces do not cause a change in motion. (If net force is 0, no change
in motion)
 Know the general idea behind each of Newton’s Laws of Motion (deals with forces)
o Law #1 – Inertia, an object resists a change in motion unless a force acts on it
 Inertia is related to the mass of an object – more mass – more inertia! The
more inertia something has the harder it is to stop its motion, or harder it is
to start it moving!
o Law #2 – Acceleration F=ma (Force=mass x acceleration)
 If the force acting on an object changes, so does the acceleration of the
object Force Up-Acceleration Up, Force Down-Acceleration Down
 If mass is different, but use the same force, the smaller or less massive
object will move faster!
o Law #3 – For every action (force), there is an opposite and equal reaction (force)
 Rocket ships push down with a blowing force to move up
 Your hand slapping a table with 25N of force will result in the table applying
25N of force back on your hand.
 Be able to use the formula F=ma to find Force, mass or acceleration, depending upon
the other 2 variables given.
 Momentum is mass in motion p=mv (mass x velocity) Increase mass, you increase
momentum. Increase velocity you increase momentum. Can a small (smaller mass)
object have more momentum than a large (larger mass) object? Sure! How?
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 Terminal Velocity - When in free fall, the force of the wind resistance pushing up
equals the force of gravity pulling you down and you just fall at a constant speed – no
more acceleration from gravity.
 Elastic Force – The kind of force you get from a wind-up toy, the elastic can pull and
cause motion